Excitation-contraction (EC) coupling and calcium (Ca2+) cycling play an important role in regulating cardiac contractile force and in the development of cardiac diseases. Disturbance of Ca2+ handling occurs at multiple levels in heart failure and is closely related to pathological performance. However, there are limited means to evaluate alterations in EC coupling in vivo. Recent studies have indicated the role of neuronal NOS (nNOS) in regulating EC coupling, with several aspects of nNOS modulation of myocardial contractility and its role in cardiac diseases still poorly understood. In the heart, nNOS has been reported to be associated with the sarcolemma, sarcoplasmic reticulum (SR), and mitochondria. Co-localization of nNOS with its effector proteins has been suggested to be important mechanisms in myocardial control. However, studies that employ a global nNOS knockout model, the NOS1-/- mouse, cannot address the complexities of NO action through spatial confinement. Therefore, the objectives of the proposed research are 1) to develop manganese-enhanced magnetic resonance imaging (MEMRI) methods for in vivo characterization of Ca2+ uptake in myocardium, the first-step in Ca2+ cycling; 2) to apply state-of-the-art MRI technology to the investigation of the differential roles of nNOS in the regulation of cardiac function and the development of cardiomyopathy. We will characterize two mouse models that differ in nNOS disruption, i.e., the global nNOS knockout mouse and the 1-dystrobrevin knockout mouse, which leads to the disruption of nNOS in cell membrane only. By combining in vivo MRI characterization of cardiac phenotypes such as function and Ca2+ uptake with in vitro molecular/cellular analysis of myocyte contractility and Ca2+ cycling in a systematic comparative study of novel mouse models with distinctive modes of nNOS disruption, this approach offers unique opportunity for dissecting the roles of nNOS in regulating cardiac function in distinct subcellular compartments. The mechanistic elucidation of the effects of nNOS on myocardial contraction and disease progression will allow nNOS to be a therapeutic target in cardiovascular diseases.